Black ice is believed to be the cause of a freeway pileup involving more than a dozen tractor-trailers that left 12 people injured Saturday in eastern Oregon, police said.

The 26-vehicle pileup left a long stretch of Interstate 84 closed for most of the day as crews worked to clean spilled diesel fuel and clear away dozens of vehicles that were either damaged or stranded.

12 people were treated for injuries at a Baker City hospital, with four of them then going to other hospitals with more serious injuries, Oregon State Police Sgt. Kyle Hove said. Police had initially reported just four people hurt. There were no fatalities.

The crash occurred shortly before 5 a.m. east of Baker City, about 130 miles northwest of Boise, Idaho.

Hove arrived on the scene to find “a sheet of ice” on the roadway. Troopers are still investigating, but Hove said he believes it was a chain-reaction crash.

“A couple of the trucks came upon the black ice, and they jackknifed and crashed into each other. And it just continued to escalate,” Hove told The Associated Press by phone.

Authorities updated their report hours after the crash, increasing the number of vehicles directly involved from 20 to 26, and saying that the pileup left 50 to 70 vehicles stranded at the scene.

There were several separate collisions over a stretch of about three-quarters of a mile in the eastbound lanes, Hove said. He said many of the damaged vehicles were tractor-trailers, and several spilled their loads.

Two trucks were transporting hazardous materials. Their cargo did not spill, but crews had to clean up diesel from the trucks’ fuel tanks.

“We understand it’s frustrating when the freeway is closed like this,” said Tom Srandberg, a spokesman for the Oregon Department of Transportation. “Our primary concern is safety, and (we) want to make sure everything is cleared by our hazmat folks before we let any traffic through the areas.”

Interstate 84 links Salt Lake City with Portland, Oregon, and is the primary east-west highway through eastern Oregon. The eastbound lanes were closed for several hours but officials said at least one lane was reopening Saturday night. Westbound lanes were closed temporarily near the crash site.